1. Field of the Invention
The present invention relates to a weld metal of high-strength Cr—Mo steel formed by welding using a covered electrode.
2. Description of the Related Art
Large heat-resistant containers of Cr—Mo low alloy steel intended for use in high-temperature, high-pressure environment, such as environment in power plants and chemical plants, have thick walls. Those containers are made of high-strength Cr—Mo steel containing V, Nb and such. Steel materials of high-strength Cr—Mo steel are cut in desired shapes. The shaped high-strength Cr—Mo steel materials are welded together to build a large container.
Generally, those high-strength Cr—Mo steel materials are welded by efficient submerged arc welding. Shielded metal arc welding is needed for welding nozzles and pipes. Improvement of the heat resistance (high-temperature strength), stress relief (SR) cracking resistance (resistance to intercrystalline cracking during annealing for stress relieving), toughness, and temper embrittlement resistance of a weld metal in a weld zone is desired. Various techniques for the improvement of toughness and temper embrittlement resistance have been proposed. A technique relating to a TIG welding wire is disclosed in Jpn. Pat. No. 2742201. This technique intend to improve temper embrittlement resistance by limiting a parameter represented by (Si+Mn)/C (P+Sn+Sb+As) to a fixed value. A technique relating to weld metal and covered welding materials is disclosed in Jpn. Pat. No. 3283773. This technique improves SR cracking resistance by suppressing the precipitation of cementite in prior austenite grain boundaries through the control of the composition of a residual obtained by electrolytic extraction such that the residual contains 35 wt. % or below Fe and 10 wt. % or above V. A technique relating to the improvement of temper embrittlement resistance is mentioned in JP-A H1-271096. This technique improves temper embrittlement resistance by reducing the oxygen content of a weld metal produced by submerged arc welding.
The efficiency of TIG welding work is low. Highly efficient welding methods, such as shielded metal arc welding methods, gas shielded arc welding methods and submerged arc welding methods, are desirable for welding recent large structures having a big wall thickness. The oxygen content of a weld metal produced by welding methods other than the TIG welding method is inevitably high. Therefore, the composition mentioned in Jpn. Pat. No. 2742201 needs to be examined. Although all the characteristics of the weld metal formed by shielded metal arc welding mentioned in Jpn. Pat. No. 3283773 are well balanced, the temper embrittlement resistance of this weld metal needs improvement. The effect of the submerged arc welding method mentioned in JP-A H1-271096 on the improvement of temper embrittlement resistance is not satisfactory.